Conventionally, subscribers are connected to exchanges via subscriber connection units. Each connection unit is connected to an exchange via telephone channels and a No. 7 channel for conveying signalling in compliance with CCITT protocol No. 7. Connection units may be situated close to their parent exchange or at a certain distance therefrom.
Calls between two subscribers necessarily pass via the exchange even though that constitutes an elongation of the path followed by such calls whenever a call is to be established between two subscribers connected to the same unit, or between two units that are very close together. An article entitled (in translation) "The exchange, master ISDN component" published in "Communication et Transmission", No. 3, 1987, page 73 et seq., describes a connection unit (a digital satellite exchange) of the Alcatel E10 system. An article entitled (in translation) "AXE 10 in France" published in "Commutation et Transmission", No. 4, 1988, page 54, describes another subscriber connection unit. Both of those connection units include emergency functions providing independent routing ability for local calls and for use in the event of the semaphore channel to the parent exchange being interrupted. However these functions are very limited. The connection units are not capable of communicating with other nearby connection units, nor are they capable of communicating with local exchanges that are closer than the parent exchange. Local calls are thus always routed in a way that makes use of transmission resources between each connection unit and its parent exchange, even though the shortest possible path would not necessarily go via the parent exchange.
The architecture of present networks is thus the result of a compromise between an architecture comprising few exchanges with connection units that are remote from the exchanges, thereby minimizing switching costs while maximizing transmission costs, and an architecture comprising many exchanges with connection units close to the exchanges, thereby minimizing transmission costs while maximizing switching costs.
An object of the invention is to provide a switching unit that overcomes this dilemma, i.e. which enables transmission costs to be reduced without increasing the number of exchanges.